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Question:

Which of the following processes results in the formation of the new compound?

A. Dissolving common salt in water
B. Heating water
C. Heating of the platinum rod
D. Heating iron rod

I've marked Option A but when all the questions and answers are officially published, it says Option D is correct.

I've studied that Heating of iron is a just physical change. But how does it forms a new compound? I tried to look it on google. I found this link which says Option D is correct. How?

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    $\begingroup$ The question is incomplete in that it does not specify in what environment the iron rod is heated - in vacuum, in inert atmosphere, in oxygen? It can be argued that heating the iron rod in oxygen (or air) results in surface oxidation, creating oxides of iron. $\endgroup$
    – Waylander
    Commented Jun 30, 2022 at 7:54
  • $\begingroup$ Heating a element cannot lead to anything but that element. Even at T common in cores of massive exploding stars, it is difficult and energy consuming to form heavier elements. // A asking B why C provided a nonsense or implicitly conditioned answer does not make much sense. Ask C or related resources instead. // BTW enough heating of water leads to H2 and O2. $\endgroup$
    – Poutnik
    Commented Jun 30, 2022 at 8:23
  • $\begingroup$ @poutnik maybe heating water does not produce H2 and O2. The decomposition temperature could be high enough to dissociate one or the other into single atoms. $\endgroup$
    – Oscar Lanzi
    Commented Jun 30, 2022 at 10:03
  • $\begingroup$ @Oscar Perhaps O, but there is still some way to H. Sure, all depends on considered eq. constants. $\endgroup$
    – Poutnik
    Commented Jun 30, 2022 at 10:23
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    $\begingroup$ The question is underspecified and the complete conditions are not given. But, if we assume common lab conditions and not extreme ones, the first three options are obviously false. Only the heating of iron under normal conditions could result in a new compound (from oxidation in a normal atmosphere). $\endgroup$
    – matt_black
    Commented Jul 1, 2022 at 10:02

2 Answers 2

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Hot iron is slowly oxidized in the presence of air, producing $\ce{Fe3O4}$ . It is mainly a surface effect, and the reaction is not stoichiometric. So the answer D is correct, if the hot iron rod is maintained a long time in air.

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Presumably heating in air is meant, in which case iron does oxidize typically forming multiple oxides. At lower temperature we would get predominantly $\ce{Fe3O4}$ with a thinner layer of $\ce{Fe2O3}$ on top, the latter being the oxide that is actually equilibrated with air (but not simultaneously with the metal). Higher heating temperature results in the formation of $\ce{FeO}$ or more accurately $\ce{Fe_{1-x}O}$, which then becomes kinetically dominant.

It turns out that platinum can be oxidized to $\ce{PtO2}$ on heating in air or pure oxygen, up to a temperature of 500°C where this oxide decomposes again [1]. Thus technically, both (C) and (D) are correct if heating is done in air.

Reference

Chaston, J.C. (1964). "Reaction of Oxygen with the Platinum Metals" (PDF). Platinum Metals Rev. 8 (2): 50–54.

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